1. Field of the Invention
This invention generally relates to an information storage apparatus, and, in particular, to an optical disk drive including an optical disk for storing information thereon.
2. Description of the Prior Art
Recently, there has been proposed various high density memory storage apparatus employing an optical recording medium on which information is stored with the use of a light beam, such as a laser beam. One typical example is an optical disk drive using an optical disk as a recording medium. Generally speaking, an optical disk drive includes an optical disk as a means for storing information thereon, driving means for driving to rotate the optical disk in a predetermined direction, an optical pick-up provided with a light source and an optical system for directing a light beam to the surface of the optical disk and positioning means for positioning the optical pick-up relative to the optical disk in the radial direction thereof.
In the case of an optical disk drive, an optical disk provided with either a single spiral recording track or a plurality of concentric recording tracks is driven to rotate in a predetermined direction and a laser beam modulated with information to be recorded is directed to the surface of the optical disk, whereby a series of pits is formed on the optical disk along the recording track in accordance with the information to be recorded. The track of the optical disk typically has the width of approximately 1 micron and also the track pitch of approximately 2 microns, and the size of the beam spot formed on the optical disk is also in the order of 1 micron. On the other hand, in the read out mode, a laser beam of predetermined intensity is directed to a desired track of the optical disk in rotation, whereby the reflecting light from the optical disk is detected to retrieve the information recorded on the optical disk. In the case where pits are formed as described above, the optical disk may be written only once. However, if use is made of other means, such as local inversion of the direction of magnetization, for storing information, the information written on the optical disk may be erased and the optical disk may be used for writing in of another information, in which case the optical disk is erasable.
The optical pick-up typically includes a semiconductor laser for emitting a laser beam, a collimator lens for receiving the laser beam from the laser and having it collimated as it passes therethrough, an objective lens for receiving the collimated laser beam after passing through a beam splitter and causing it to be focused on a recording track defined on the recording surface of the optical disk. Typically, a quarter wavelength plate is also provided between the objective lens and the beam splitter. Then the reflecting laser beam from the optical disk again passes through the objective lens this time in the opposite direction and then the quarter wavelength plate and then it is deflected by the beam splitter into a detecting optical system which typically includes a tracking error detector and a focusing error detector. Thus, the deflected laser beam is received not only by the tracking error detector but also by the focusing error detector so that there are obtained a tracking error signal and a focusing error signal, which are supplied to a tracking servo controller and to a focusing servo controller, respectively, which, in turn, define part of the relative positioning means between the optical disk and the optical pick-up.
The optical pick-up is typically provided with a tracking actuator for moving the objective lens in the widthwise direction of the track and with a focusing actuator for moving the objective lens closer to or separated away from the optical disk. These tracking and focusing actuators are activated under the control of the respective tracking and focusing servo controllers so that the objective lens is moved so as to bring the tracking and focusing errors to zero, whereby the relative positioning between the objective lens and a particular recording track of the optical disk is established and a beam spot of predetermined size is formed on a particular recording track of the optical disk. In this case, if the objective lens is moved over a relatively large distance in the tracking direction, i.e., widthwise direction of the recording track, in accordance with a signal from the tracking servo controller, the optical axis of the laser beam deviates significantly from a predetermined position. For this reason, there is a case in which the position of receiving the laser beam at the detector is shifted too far, thereby preventing to obtain read-out and error signals properly. Thus, in order to prevent from this happening, it is typically so structured that the optical pick-up itself moves in association with the movement of the objective lens so as to prevent the receiving position of the laser beam at the detector from shifting too far.
In moving the optical pick-up in this manner, the response speed of the system when a drive signal corresponding to the amount of movement in association with the timing of movement of the objective lens is supplied must be high. For this purpose, as a driving source for driving to move the optical pick-up in the tracking direction, use is typically made of a linear motor which directly generates linear motion and which is high in responsiveness. Thus, in a compact optical disk drive, use is typically made of a linear motor having a structure in which a moving coil is provided on a carriage on which the optical pick-up is mounted and a magnetic circuit member, such as a permanent magnet, is provided on a chassis. In such a typical linear motor or actuator, a pair of guide shafts is provided in parallel with the tracking direction and each of the guide shafts is sandwiched by a plurality of rollers rotatably mounted on both sides of the carriage, thereby providing a smooth guide to the carriage in the tracking direction.
In such a typical prior art structure in which the linear motor is mounted at the bottom surface of the carriage, the linear motor is necessarily disposed in the rear of the optical pick-up which extends through the carriage. For this reason, the thrust center, i.e., the center position where the thrust of the linear motor is produced, is located far apart from the center of gravity of the carriage, which is located in the interior of the optical pick-up having the largest mass among the components of the linear motor. In this manner, with the thrust center being separated far away from the center of gravity, a moment is generated when the carriage is moved so that there appears a pitching motion in the carriage. Besides, as described above, since the linear motor is operated finely in synchronism with the movement of the objective lens in the tracking direction, such a pitching motion has a relatively high frequency component.
In addition, as described above, the carriage is supported on the guide shafts via a plurality of rollers, so that when the front portion of the carriage moves downward for some reason at the location where the front rollers are provided by the amount of play, the rear portion of the carriage moves upward at the location where the rear rollers are provided by the similar amount of play. In other words, the carriage could move up and down by the amount of play of the rollers. The resonant condition of such an up and down vibration of the carriage has a relatively large frequency, and, thus, when the carriage is set in a resonant condition with the frequency component of the before-mentioned pitching motion, the amplitude of the up and down motion is significantly amplified, whereby the tracking servo and focusing servo controls are adversely affected. Besides, since the optical pick-up and the linear motor are mounted on the top and bottom surfaces of the carriage, respectively, the optical disk drive tends to have a large thickness, thereby hindering to make the overall structure compact in size.